Flatness defects of plates are roughly classified into shape defects and warp defects. Warp defects can be corrected by a conventional roller leveller using cylindrical rollers. Shape defects are formed by differences of elongation in a longitudinal direction of a plate, and cannot be corrected unless plastic elongation or shrink is given to the plate in the longitudinal direction. Of such shape defects, a large curve of a plate in the lateral direction is called a camber.
A conventional roller leveller uses cylindrical rollers disposed in a zigzag way as upper and lower roller groups and repeatedly applies a bending force to a plate or coil plate (plate) to remove distortion. This roller leveller can correct warp defects of a plate.
Various methods have been proposed in order to remove a camber of a plate (for example, refer to Japanese Patent Publication No. 7-29137 "Camber Straightener", Japanese Utility Model Laid-open Publication No. 4-17310 "Straightener for Camber", and Japanese Patent Publication No. 61-54484 "Roller Leveller"). However, all of these methods require to change settings of a roller leveller depending upon whether the direction of a camber is right or left.
A tension leveller by which a steel coil is processed with tension being applied is effective for correcting a camber of a steel coil. The tension leveller can correct the shape of a workpiece (coil plate) by applying plastic elongation to it in the longitudinal direction. However, although the tension leveller can be applied to a relatively thin coil plate, it cannot be applied to a thick coil plate and a sheet.
It is conceivable that a camber of a wide plate can be corrected by applying a bending moment to the plate in a lateral direction in its plane. However, if a camber correcting bending moment is applied to a thin and wide plate, torsional buckling of the plate occurs before a sufficient bending moment is applied. The bending moment sufficient for correcting the camber cannot therefore be applied.
This bending work for camber straightening is unstable and cannot be used in practice for straightening the camber (for example, refer to "Strength of Materials" by Kansei Ono, published by Maruzen Publishing Co., in Item of Thin Beam, at p. 156). A conventional roller leveller cannot straighten a camber of a plate, although it can correct warp defects of a plate.
It is difficult to straighten a camber. Instead of straightening a camber, both lugs of a plate in the longitudinal direction are cut out or cut away to narrow the plate width and remove the camber. With this method, however, a manufacture yield of plates is reduced.